Is the device listed in Device Manager and with no red exclamation or yellow question-mark beside the entry?
If there is such a mark it denotes a problem with the driver either being corrupted or missing, in which case the drivel needs to be re-installed (a driver is simply a small bit of software that tells Windows what the device is and allows it to work).
All device drivers are different and are made by the manufacturer. Your soundcard is soldered on to the motherboard (known as "on-board" or "integrated" sound), so we need to know the motherboard make and model.
Download 'Belarc Advisor" from here:
http://www.belarc.com/free_download.html
No need to install it. Just double-click the downloaded file and it will produce a report of your system details. Post those details to us and we'll see what it says.